BY J. MITCHELL. 853 



extremities, whence it turns rather abruptly backwards. Axis 

 moderately convex, consisting of two wide rings, indefinitely 

 separated. Axial grooves shallow. Side lobes depressed, 

 pleural divisions indistinct, but they are each divided into two 

 lulus by faint ridges. Border strong, thickened, carries eight 

 or doubtfully ten spines, two being intermediate to the strong 

 pair . 



Obs. — This trilobite in some particulars resembles both 0. 

 rattti E. and M. and 0. jenkinsi E. and M., but is more nearly 

 related to the former than the latter, and this relationship is 

 very noticeable in comparing their pygidia; but even in these 

 parts material differences are readily observable. For instance, 

 the tail spines in the present are eight and doubtfully ten at 

 most; in 0. rattei they number twelve. The two strong spines 

 in the latter species, subtend from strong pleural ridges, but 

 in the former weak ones. The wings of the fixed cheeks in 0. 

 hartleii too are relatively longer and slenderer than in 0. rattei 

 or in 0. jenkinsi. The central part of the glabella in O. rattei 

 is relatively longer and more convex than in the present species. 

 The limb is much more distinctly dentate in the former than in 

 the latter; in the latter, also, the antero-lateral angles of the 

 glabella are much more expanded than in the other. The neck 

 furrow and ring's in O. rattei are well developed; the opposite 

 is the case in the present form. 



Dedicated to Mr. Charles Hartley of Balheary, Chatsbury, a 

 member of one of the pioneering families of the Goulburn dis- 

 trict and who collected the specimens above described. 



Loc. and horizon. — Back Creek, Tarlo River, Parish of Tur- 

 rallo, County Argyle, one mile N.W. of the slate quarry, asso- 

 ciated with undetermined Encrinurus. Probably Upper Silurian. 



Order KYPOPARIA. 

 Family TRJNUCLEIDAE. 



TRIXUCLEUS CLARKEI, n. Sp . 



(Plate xlvii., figs. 12-14.) 



A pygidium obtained from Duntroon associated with Encri- 

 nurus duntroonemis, I have determined to belong to the 

 above genus. The following are its chief features: — Widely 

 triangular, mildly convex and apparently had been granulated; 



